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LAUREL, Mont. — Teams of federal and state workers fanned out yesterday along Montana’s famed Yellowstone River to gauge the environmental damage from a ruptured Exxon Mobil pipeline that spewed tens of thousands of gallons of crude oil into the waterway.

The break near Billings, in south-central Montana, fouled the riverbank and forced municipalities and irrigation districts to close intakes. An estimated 1,000 barrels, or 42,000 gallons, spilled Saturday before the flow from the damaged pipeline, which is in the riverbed, was stopped.

Sonya Pennock, a spokeswoman for the Environmental Protection Agency, said an unspecified amount of oil could be seen some 40 miles downriver during a fly-over yesterday, and there were other reports of oil as far as 100 miles away near the town of Hysham.

But an Exxon Mobil Corp. executive said shoreline damage appeared to be limited to the Yellowstone between Laurel and Billings, which includes about 20 miles of river.

Gary Pruessing, president of the Exxon Mobil Pipeline Co., said company observers flying over the river had seen “very little soiling’’ beyond Billings, and that the oil appeared to be evaporating and dissipating into the river as the flooded Yellowstone carries it downstream.

A representative of the Montana Disaster and Emergency Services Division said the company’s claim was reasonable but had not been independently verified.

On Saturday, state officials had reported a slick headed downstream toward the Yellowstone’s confluence with the Missouri River, just across the Montana border in North Dakota.

“My guess is that as fast as that water is moving, it’s probably dissipating pretty quick,’’ said DES public assistance officer Tim Thennis.

Exxon Mobil also revealed yesterday that the 12-inch pipeline had been temporarily shut down in May because of concerns over the rising waters on the Yellowstone. Pruessing said the company decided to restart the line a day later after examining its safety record and deciding the risk of failure was low.

The company and government officials have speculated that high waters in recent weeks may have scoured the river bottom and exposed the pipeline to debris that could have damaged the pipe. The state has received record rainfall in the last month and also has a huge snowpack in the mountains that is melting, which has resulted in widespread flooding.

An EPA representative said only a small fraction of the spilled oil is likely to be recovered. Agency on-scene coordinator Steve Way said fast flows along the flooding river were spreading the oil over a large area, making it harder to capture. But Way said that also could reduce damage to wildlife and cropland along the river.

Crews were putting absorbent material along short stretches of the river in Billings and near Laurel, but there were no attempts at capturing oil farther out in the river. In some areas, oil flowed underneath booms and continued downstream.

Up to 100 emergency response workers from Exxon Mobil and its contractors were sent to the scene. Pruessing said they would remain there until the cleanup is complete.

But property owners along the river were growing frustrated with the response, particularly in agricultural areas where crops and pastures for grazing were at risk. The Yellowstone River is also popular among fishermen, though areas farther upriver from the spill are more heavily fished.

Billings-area goat rancher Alexis Bonogofsky said the flooding Yellowstone brought the oil into her summer pastures. Bonogofsky said she had been unable to get answers through government authorities or Exxon Mobil.

“My place is covered with oil,’’ she said. “I would like a list that says ‘this is what’s in crude oil.’ . . . I called a million times yesterday and got no response.’’

The 20-year-old pipeline was last inspected in 2009 using a robotic device that travels through the line looking for corrosion, dents, or other problems, Pruessing said. Soundings to determine the pipeline’s depth were taken in December, and at the time, the line appeared to be 5 to 8 feet below the riverbed, he said.

The estimated 42,000 gallons spilled was a small fraction of that in major accidents; 11 million gallons were spilled in Alaska’s Exxon Valdez disaster in 1989, for example. But officials said the pristine nature of the Yellowstone, along with its turbulent waters and riverside communities, were likely to make for unique challenges as cleanup and damage assessment progressed.